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Abstract:

Disclosed is a display for the holographic reconstruction of a
three-dimensional scene using means which allow a reduction of speckle
patterns. Speckle patterns result in the graining of a holographic
reconstruction and worsen the quality thereof. The 3D scene is
incoherently superimposed with itself chronologically or spatially in the
eye of the observer. The modulated wave fronts of each reconstructed
object point of the scene are shifted relative to themselves in the
reconstruction beam path and superimposed in the eye of the observer. The
shifting may occur one-dimensionally and two-dimensionally. Each object
point is multiplied with itself in the eye of the observer in accordance
with the number of the shifted wave fronts. The various speckle patterns
over which the eye of the observer averages are also multiplied. Speckle
patterns are reduced and the reconstruction quality is thus increased in
holographic displays.

Claims:

1. Holographic display device for reconstructing a scene which is divided
by software means into object points, withsystem controller means for
computing a computer-generated hologram (CGH) of the scene and for
encoding the CGH on a light modulator means,a light source means for
illuminating the light modulator means with coherent light, anda
reconstruction means for transforming the light in the form of modulated
wave fronts into an observer window, from where at least one observer eye
sees the holographic reconstructions of the object points which are
generated by the modulated wave fronts in a reconstruction space that
stretches between observer window and light modulator means and which are
superimposed in the observer window, whereinthe reconstruction beam path
comprises means for temporally or spatially displacing the modulated wave
fronts emitted by the light modulator means of each object point which is
reconstructed in the reconstruction space such that the reconstruction
means multiplies each reconstructed object point with itself caused by
this displacement, and that the multiplied reconstructions of each object
point are incoherently superimposed with themselves in the observer
window.

2. Holographic display device according to claim 1, wherein the
multiplication of the reconstruction of each object point is performed at
least twice, in two perpendicular directions.

3. Holographic display device according to claim 1, wherein a mirror is
provided which is disposed at a given angle to the optical axis of the
light modulator means, and which can be moved both laterally and along
the optical axis.

4. Holographic display device according to claim 1, wherein a prism matrix
is provided in a plane which is parallel to the light modulator means,
where said prism matrix can be moved both along the optical axis of the
light modulator means and laterally.

5. Holographic display device according to claim 1, wherein a variably
controllable prism pair is provided in a plane which is parallel to the
light modulator means, where the angles of refraction of the prisms
sequentially vary between at least two values at a high switching
frequency to realise a very fast displacement of the modulated wave
fronts.

6. Holographic display device according to claim 1, wherein a matrix of
controllable prism pairs is provided in a plane which is parallel to the
light modulator means, where the angles of refraction are sequentially
controllable variably between at least two values at a high switching
frequency to realise a very fast displacement of the modulated wave
fronts.

7. Holographic display device according to claim 1, which is a projection
display, where a variably controllable prism is disposed centrally in a
Fourier plane, which is at the same time the front focal plane of an
optical reconstruction system.

8. Holographic display device according to claim 1, wherein a matrix of
rhombic prisms is assigned to the light modulator means in combination
with a polarisation switch or the light modulator means is followed by a
first optical component made of a birefringent material in combination
with a polarisation switch, after which a second optical component made
of a birefringent material is disposed.

9. Holographic display device according to claim 8, where a combination of
two matrices of rhombic prisms and two polarisation switches (PU) are
provided for a two-dimensional displacement.

10. (canceled)

11. Holographic display device according to claim 8, wherein the
polarisation switch is an active or a passive element.

12. Holographic display device according to claim 1, wherein a combination
of two Bragg gratings with spacer in between is provided in the
reconstruction beam path for a one-dimensional displacement of the
modulated wave fronts, where each Bragg grating has a defined grating
geometry.

13. Holographic display device according to claim 12, wherein the
combination of Bragg gratings and spacer additionally comprises a
90.degree. polarisation switch in order to realise a sequential
two-dimensional displacement of the modulated wave fronts.

14. Holographic display device according to claim 12, wherein the Bragg
gratings are combined with a 45.degree. polariser in order to split the
modulated wave front of each reconstructed object point into two
perpendicular components and to displace them one-dimensionally against
one another simultaneously.

15. Holographic display device according to claim 12, wherein for a
two-dimensional displacement of the modulated wave fronts of the object
points at least one Bragg grating is written to a volume hologram for one
direction and at least one Bragg grating is written to a volume hologram
for another direction.

16. Holographic display device according to claim 15, wherein two volume
holograms with Bragg gratings written to them are arranged in relation to
each other such that for each two-dimensionally multiplied object point a
resultant pattern is generated in that always two adjacent object points
are superimposed such that they exhibit perpendicular polarisation p and
s, so that they can be reconstructed incoherently to each other.

17. Holographic display device according to claim 12, wherein the
combination of the Bragg gratings with a spacer is provided for each
colour, in order to achieve a colour reconstruction of the scene with the
three primary colours RGB.

18. Holographic display device according to claim 17, wherein the
combination of the Bragg gratings with the spacer is written to a volume
hologram such that the volume hologram includes always two Bragg gratings
per direction and per colour, in order to achieve a colour reconstruction
of the scene with the three primary colours RGB.

19. Holographic display device according to claim 9, wherein the
polarisation switch is an active or a passive element.

Description:

[0001]The present invention relates to a display device for
holographically reconstructing a three-dimensional scene, said display
device having an improved reconstruction quality thanks to the reduction
of speckle patterns.

[0002]This invention can for example be applied in holographic display
devices which are used to generate, store and reconstruct holograms of
the three-dimensional scene in real-time or near-real-time processes with
the help of coherent laser light. The reconstruction of the scene in such
a display device is visible through a visibility region, which is also
referred to as observer window, in a reconstruction space.

[0003]The method for reconstructing a scene, where the reconstruction is
visible through an observer window, and examples for the computation and
encoding of the hologram of the scene have been described in earlier
documents filed by the applicant, for example in (1) EP 1 563 346 A2 and
(2) DE 10 2004 063 838 A1.

[0004]Further, those documents describe a holographic display device in
which the above-mentioned method for the reconstruction of a hologram is
implemented. The reconstruction method will be briefly explained below:

[0005]For the holographic reconstruction, a three-dimensional scene is
sliced by software means into section layers, each of which comprising a
multitude of object points of that scene. The object points characterise
both the section layer and, as the sum of all layers, the
three-dimensional scene.

[0006]A computer-generated hologram (CGH) is computed based on the object
points as a two-dimensional arrangement of generally complex values,
which are represented on a light modulator means. The light modulator
means comprises regularly arranged, controllable elements for the
modulation of the wave fronts of the incident coherent light with the
complex values of the encoded scene. The reconstruction of the scene is
generated in a reconstruction space with the help of the coherent light
and a reconstruction means, which is controlled by system controller
means. The wave fronts of the reconstructed object points are coherently
superimposed in the observer window. An observer sees from an eye
position in that observer window the resultant reconstruction of the
scene in the reconstruction space, which stretches between the observer
window and a modulator means or screen.

[0007]According to a modified version of this method, a reconstruction of
the scene can also be generated by computing individual CGHs from the
individual object points, and by encoding separate regions on the light
modulator means with those sub-holograms. The phase distribution of the
complex values in the region of the sub-hologram roughly corresponds with
a holographically encoded lens function, which reconstructs that single
object point in its focal point. The focal length of such a lens depends
on the axial distance of the object point from the light modulator means
or screen.

[0008]The absolute value of the complex values, i.e. the amplitude, is
about constant across the entire sub-hologram, and its magnitude depends
on the axial distance of the object point from the screen or light
modulator means, and on the brightness of the object point. As coherent
light passes through the light modulator means, the complex transparency
values which are encoded there modify the amplitude and/or phase of the
light. The object point is reconstructed by the modulated light. Outside
the sub-hologram, this object point has the value zero on the light
modulator means, i.e. it is only represented by the sub-hologram. The
total encoded hologram of the scene is generated by adding the complex
values of the individual sub-holograms.

[0009]According to a simplified version of the method, object points are
for example combined according to certain criteria so to form object
point groups, where each group is represented by one CGH in a sequential
process. Their wave fronts are in that case superimposed incoherently in
the observer window and generate a resultant reconstruction of the scene
in the reconstruction space.

[0010]This is described with several computation and representation
options in hitherto unpublished documents filed by the applicant, e.g. in
DE 10 2006 062 377 and DE 10 2007 023 738.

[0011]For watching the reconstruction of the three-dimensional scene, the
observer can either look at a light modulator means on which a hologram
of the scene is directly encoded, and which serves as a screen. This is
referred to as a direct-view arrangement. Alternatively, the observer can
look at a screen onto which an image of the hologram values encoded on
the light modulator means is projected. This is referred to as a
projector arrangement.

[0012]The eye positions of observers are detected by a position finder in
a known manner, said position finder being linked by software means with
a storage means and a computing unit, and with a system controller means.
The storage means also hosts the information of the object points which
are necessary for computing the CGH in data records in the form of a
look-up table.

[0013]The size of the observer window in front of a display means is
defined; it is typically as large as an eye pupil. Seen from the
wave-optical point of view, an observer window is formed either by a
direct or inverse Fourier transform or Fresnel transform of a hologram
encoded on a light modulator means, or by the image of a wave front
encoded on a light modulator means in a plane of a reconstruction space.
The observer window only comprises a single diffraction order of a
periodic reconstruction of the scene. The plane may be a focal plane of a
focussing means, or the image plane of a light source. The hologram or
the wave front are computed from the scene such that, within the one
diffraction order which serves as the visibility region, cross-talking
between the observer eyes is prevented, which would typically occur in
reconstructions when using light modulators. In conjunction with an
arrangement or a method for suppressing higher diffraction orders, scenes
can be consecutively presented in a multiplex process to a left and to a
right eye of an observer without any cross-talking. Moreover, a multiplex
process with the aim to serve multiple persons only then becomes
possible.

[0014]The pixels of spatial light modulators, such as LCD, LCoS etc.,
which modulate the phase and/or amplitude of incident light, serve to
represent the holograms and to generate the complex-valued wave fronts of
the scene. The refresh rate of the light modulator means must be
sufficiently high in order to be able to represent a moving scene.

[0015]Because of the coherence of lasers, disturbing patterns, which are
also known as speckle patterns or granulation, occur in the observer
plane when using laser light for illuminating a light modulator. Speckle
can be described as a granulation-like interference pattern which is
created by interference of multiple light waves with statistically
irregularly distributed phase differences. It disturbs the observer in
watching the reconstruction of the scene, and it causes spatial noise
there.

[0016]Speckle patterns can generally be reduced by temporal and/or spatial
averaging of reconstructions of the scene in the observer eye. The
observer eye always averages out multiple reconstructions with different
speckle patterns presented to it, which results in a smoothing of the
contours of the reconstructed scene.

[0017]According to document DE 195 41 071 A1, for example, a rotating
glass plate is put into the optical path in order to temporally average
the granulation or speckle patterns when checking the authenticity of a
hologram. It rotates at a frequency which matches the frequency of a
detector used for recording. Speckle patterns do thus not occur as
disturbing effects.

[0018]However, such a method can only be applied for reducing
two-dimensional, plane speckle patterns, where the diffusing plate must
be disposed in the plane of the speckle patterns. The disadvantage of
this method is that too much light is lost because of a diffusing plate
in the light path. Further, it shall be avoided to use a mechanically
rotating component in designing a holographic display device.

[0019]Another known method of reducing speckle patterns is to compute the
scene with a given number of different random phases, and to represent
the resultant holograms on a light modulator means one after another at a
fast pace. However, the computational load increases substantially
because of the many hologram computations. Further, a light modulator
means provided to represent the holograms must have a very fast refresh
rate.

[0020]It is the object of the present invention to realise methods based
on averaging processes for reducing speckle patterns in a holographic
display device with an observer window using simple means and without
much effort. Light source means which generate coherent light and
commercially available light modulator means shall be used in doing so.
It is not necessarily required to use fast-switching light modulator
means.

[0021]The present invention for reducing speckle patterns takes advantage
of methods which are based on averaging multiple reconstructions of the
three-dimensional scene by observer eyes, which have already been
described by the applicant. The methods are based on the fact that a
three-dimensional scene is composed of individual objects, and these on
individual object points, whose speckle-pattern-affected reconstructions
are superimposed in the observer eye. This was described in detail in the
prior art section above.

[0022]Based on those methods, the object is solved according to this
invention in that means for temporally or spatially displacing the
modulated wave fronts of the object points are provided in the
reconstruction beam path of a holographic display device, said means
serving to multiply the reconstruction of each object point and to
incoherently superimpose the displaced modulated wave fronts in the eye
of at least one observer. An observer eye thus perceives a resultant
reconstruction with reduced speckle pattern.

[0023]According to a preferred embodiment of the invention, the
multiplication of the reconstruction of each object point is performed at
least twice, in two perpendicular directions.

[0024]The following means for displacement and superimposition according
to this invention are generally independent devices; however, they can
also be combined partly in order to solve the object. According to this
invention, they can displace and superimpose the reconstructed object
points with themselves spatially or sequentially.

[0025]In embodiments of the invention, the following ways of displacing
modulated wave fronts or reconstructed object points are possible in the
reconstruction beam path of the holographic display device:

[0026]In a first embodiment of the invention, a mirror is provided which
is disposed at a given angle to the optical axis of the light modulator
means, and which can be moved both laterally and along the optical axis.

[0027]In another physical form of the first embodiment, a prism matrix is
provided in a plane which is parallel to the plane of the light modulator
means, where said prism matrix can be moved both along the optical axis
of the light modulator means and in lateral direction.

[0028]In a second embodiment of the invention, a variably controllable
prism pair is provided in a plane which is parallel to the light
modulator means, where the angles of refraction of the prisms
sequentially vary between at least two values at a high switching
frequency. The modulated wave front of each object point can thus be
directed at an observer eye while being displaced against itself at least
once, i.e. with a lateral offset, and then be superimposed in the
observer eye. In another physical form of the second embodiment,
controllable prism pairs are provided in a regular arrangement in a
matrix for displacing the modulated wave fronts, where the angle of
refraction of the prisms sequentially varies between at least two values
at a very high switching frequency.

[0029]In a third embodiment of the invention, the display is a holographic
projection display, where a variably controllable prism is disposed
centrally in a Fourier plane, which is at the same time the front focal
plane of an optical reconstruction system.

[0030]In a fourth embodiment of the invention, a matrix of rhombic prisms
is provided next to the light modulator means in combination with a
polarisation switch. In another physical form, a combination of two
matrices of rhombic prisms and two polarisation switches is disposed next
to the light modulator means for a two-dimensional displacement of the
modulated wave fronts.

[0031]In a fifth embodiment of the invention, the light modulator means is
followed by a first optical component made of a birefringent material in
combination with a polarisation switch, after which a second optical
component made of a birefringent material is disposed. The birefringent
material is preferably of a two-part design.

[0032]The polarisation switch can be an active means in the form of a
Faraday cell, or a passive means in the form of a λ/2 plate.

[0033]In a sixth embodiment of the invention, a combination of two Bragg
gratings with spacer in between for lateral one-dimensional displacement
of the modulated wave fronts parallel to the original direction of
propagation is provided in the reconstruction beam path.

[0034]In another physical form, the modulated wave front of an object
point undergoes a sequential, two-dimensional displacement by combining
the Bragg gratings with the spacer and an additional 90°
polarisation switch.

[0035]In a further physical form, the Bragg gratings are combined with a
45° polariser in order to divide the modulated wave front of each
reconstructed object point in two perpendicular components and to
displace them one-dimensionally against one another simultaneously.

[0036]In a further physical form, each modulated wave front undergoes a
two-dimensional displacement in that at least one Bragg grating is
written to a volume hologram for one direction and at least one Bragg
grating is written to a volume hologram for another direction.

[0037]In a further physical form, two volume holograms with Bragg gratings
written to them are arranged in relation to each other such that for each
two-dimensionally multiplied object point a resultant pattern is
generated in that always two adjacent object points are superimposed such
that they exhibit perpendicular polarisations p and s, so that they can
be reconstructed incoherently to each other.

[0038]In doing so, at least one Bragg grating is required for the
multiplication of the object points in one direction.

[0039]In a seventh embodiment of the invention, the combination of the two
Bragg gratings with spacer is provided for each colour, in order to
achieve a colour reconstruction of the scene with the three primary
colours RGB. In a physical form of this embodiment, the combination of
the Bragg gratings with the spacer is written to a volume hologram such
that the volume hologram includes always two Bragg gratings per direction
and per colour, in order to achieve a colour reconstruction of the scene
with the three primary colours RGB.

[0040]The present invention will be described in detail below with the
help of several embodiments, where the FIGS. 2 to 8 are top views,
wherein:

[0041]FIG. 1 is a graphic representation of the superimposition of two
diffraction images of a single object point on the retina of the eye;

[0042]FIG. 2 is a schematic diagram which shows two reconstructed object
points in the reconstruction space, and which introduces parameters for
determining the size of a speckle pattern;

[0043]FIGS. 3a, 3b show a first embodiment for displacing the modulated
wave fronts a) with a movable mirror, and b) with a movable prism;

[0044]FIGS. 4a, 4b show a second embodiment for displacing the modulated
wave fronts with a variably controllable prism pair;

[0045]FIG. 5 shows a third embodiment with a controllable prism which is
disposed in a Fourier plane;

[0046]FIGS. 6a, 6b show a fourth embodiment a) with a matrix of rhombic
prisms in combination with a polarisation switch and b) with a
combination of two matrices of rhombic prisms and two polarisation
switches;

[0047]FIG. 7 shows a fifth embodiment with a two-part element made of a
birefringent material in combination with a polarisation switch;

[0048]FIG. 8 shows a sixth embodiment with two Bragg gratings which are
separated by a spacer; and

[0049]FIGS. 9a, 9b show two physical forms of the sixth embodiment with
sequential combination of Bragg gratings a) as a top view and b) as a
side view.

[0050]Holographic display devices according to this invention can be
realised in the form of direct-view displays or projection displays.

[0051]The invention for reducing speckle patterns is based on the general
idea that the reconstructed scene is incoherently superimposed with
itself. As the scene is composed of a multitude of object points, all
reconstructed object points must thus be superimposed with themselves.
For this, all reconstructed object points, and thus their diffraction
images, are first multiplied by displacing their modulated wave fronts in
a time or space division multiplex process, and then incoherently
superimposed in the observer eye, without changing the image content of
the scene.

[0052]Each reconstructed object point has a distinct speckle pattern. The
larger the number of superimpositions, the finer are the individual
speckle patterns averaged by the observer eye, and thus the more
significant is the perceivable reduction. The physical and software means
necessary for the reconstruction of the scene are part of system
controller means (not shown), or work together with the latter.

[0053]FIG. 1 is a graphic representation of the result of a displacement
of a reconstructed object point with the maxima and minima of the two
diffraction images. The two diffraction images are incoherently
superimposed, and their maxima exhibit a certain offset, so that a
speckle pattern is visibly smoothed.

[0054]The statistical character of the speckle patterns is determined by
the correlation length δSP, which defines the size of the
speckle pattern. For a certain section layer of the scene in the
reconstruction space, it only depends on the diameter of the eye pupil Dp
of the observer and on the wavelength λ of the light emitted by a
laser which serves as the light source means.

[0055]It can be determined with the aid of the following equation (1):

δ SP ≈ 2.4 λ S D P ( 1
) ##EQU00001##

where Dp is the diameter of the eye pupil, λ is the wavelength
and S'=f'+z' is the image width in the section layer with the images of
the object points OP1, OP2 on the retina of the eye.

[0056]If, when a reconstructed object point OP is multiplied, the distance
between its multiplied images OP' on the retina is at least as large as
the correlation length δSP, the standard deviation of the
speckle patterns of the reconstructed object points OPn will be reduced
by a factor of 2. In the case of a two-dimensional displacement of all
object points OPn, the standard deviation of the speckle patterns will be
halved.

[0057]FIG. 2 shows the geometrical and optical relations in order to
illustrate the connections.

[0058]The letter Y denotes the lateral distance in the object plane
between the object points OP1 and OP2 in the reconstruction space RK, and
the letter Y' denotes the distance between the images of those object
points in the image plane. The reconstruction space RK stretches from a
light modulator means SLM to the eye lens L; and the object points OP1
and OP2 are situated at a distance d from that eye lens L. The diameter
of the eye lens L here forms the observer window OW at the same time.

[0059]The distances Y and Y' are related as specified in equation (2):

Y = Y ' β ( 2 ) ##EQU00002##

where β is the reproduction scale, which is given in equation (3):

β=-f/z=-z'/f' (3)

where f is the focal length, z is the object width, and f' and z' are the
corresponding parameters on the image side.

[0060]It can be derived from these equations that the visible size of the
speckle patterns becomes the smaller the smaller the distance between the
observer and the reconstructed scene. Since in the reconstruction space
RK all section layers have the same number of matrix dots, where object
points OPn can be situated, the distance between the individual object
points OPn changes in proportion with the distance of the observer eye.
This means that the visible speckle pattern also changes in proportion
with the distance of the observer eye. The speckle patterns in each
section layer are thus perceived by the observer eye in the same size.

[0061]Based on that thought, FIGS. 3 to 9 show schematically possible
physical means for displacing and thus for multiplying and superimposing
the reconstructed object points of the scene, which is necessary for
reducing speckle patterns by way of averaging according to this
invention.

[0062]Lasers are used as light source means, and these lasers illuminate
the light modulator means with coherent light. The individual components
are controlled by system controller means (not shown).

[0063]In order to keep the drawings simple and comprehensible, they only
show the displacement of the wave front of one object point, which is
representative of all reconstructed object points OPn of the scene. The
wave front is shown in the form of an arrow, which indicates the
direction of displacement. Where like reference numerals are used, they
denote components which generally have like functions, unless otherwise
explained.

[0064]FIGS. 3a and 3b show two arrangements for mechanically displacing
the modulated wave fronts of reconstructed object points according to a
first embodiment of the invention.

[0065]Referring to FIG. 3a, a mirror is disposed at a fix angle of
preferably 45° to the optical axis of the light modulator means
SLM. It deflects all modulated wave fronts laterally by 90° to the
original direction of propagation. The mirror, and thus also the wave
fronts, can be displaced either one-dimensionally (laterally or along the
optical axis of the light modulator means SLM), or two-dimensionally,
i.e. in two perpendicular directions. This is indicated in the drawing by
the double arrows and by the broken lines which represent the beam path
after reflection from the mirror. Two other possible positions of the
mirror are indicated by thick square points.

[0066]However, the mirror can also be disposed at any other angle to the
light modulator means SLM, depending on the eye position of the observer,
or on other components of the holographic display device.

[0067]Referring to FIG. 3b, there is a matrix of prisms disposed in a
plane parallel to the plane of the light modulator means SLM. The prism
matrix can be displaced both one-dimensionally and two-dimensionally, as
indicated by vertical and horizontal double arrows. A displacement of the
prism matrix along the optical axis is shown in detail using the example
of a single prism. A displaced position of the prism, and thus of the
wave front, is indicated by broken lines. The wave front is deflected
towards an observer eye (not shown) at an offset to the original
direction of propagation. The individual prisms are arranged in a regular
pattern such that the prism wedges of all prisms in each row face the
same direction.

[0068]Another physical form is possible here, that is a combination of the
mirror (of FIG. 3a) and prism matrix, in order to realise a displacement.
The prism matrix is for example attached to the mirror so that they form
a compact optical unit.

[0069]The wave front of each reconstructed object point of the scene is
sequentially displaced and superimposed with itself by the arrangements
shown in FIGS. 3a and 3b. A double (one-dimensional) or quadruple
(two-dimensional) number of respective reconstructed object points is
generated and superimposed on the retina of the eye. It is also possible
to generate a larger number of displacements, if the wave fronts of the
reconstructed object points in one or in both directions are displaced
not just once, but several times. The multiple displacement is indicated
in FIG. 3a by multiple arrows which point towards the observer eye.

[0070]Referring to FIGS. 4a and 4b, a second embodiment describes an
optically effected displacement of the modulated wave fronts.

[0071]A variably controllable prism pair is disposed in a plane which is
oriented parallel to the light modulator means SLM, and a modulated wave
front of a reconstructed object point hits this prism pair. The direction
of propagation is indicated by arrows. The wave front is refracted during
its passage through the prisms, so that it is parallel translated to the
original direction of propagation. The angle of refraction of the two
prisms is switched sequentially at a high switching frequency between two
values, namely between a value according to FIG. 4a and a value according
to FIG. 4b. The displacement here effects a reconstructed object point to
be doubled, and thus to be superimposed with itself on the retina of the
observer eye; this displacement is so generated for all object points of
the scene. The diagrams shown form an example of a one-dimensional
displacement.

[0072]A two-dimensional displacement of modulated wave fronts can be
realised by designing the prisms such that they have a two-dimensionally
refracting shape, or by using two identically designed prism pairs which
are perpendicularly oriented.

[0073]It is within the scope of the present invention to use instead of a
single controllable prism pair a matrix of regularly arranged, variably
controllable prism pairs for realising the displacements.

[0074]In a third embodiment of the invention, a controllable prism is
disposed centrally in a Fourier plane of an optical transformation system
to be used preferably in a projection display.

[0075]Parallel oriented laser light illuminates the light modulator means
SLM, as shown in FIG. 5. The following optical transformation system
transforms the modulated wave fronts into the Fourier plane FE, which is
at the same time the front focal plane of an optical reconstruction
system. The controllable prism, which is centrally arranged in the
Fourier plane FE, is operated sequentially at a high switching frequency
and thus displaces the modulated wave fronts at a very fast pace. Because
of the displaced wave fronts, the optical reconstruction system
reconstructs each object point twice, with different speckle patterns. By
way of superimposing the wave fronts on the retina of the observer eye,
the eye can average different speckle patterns.

[0076]Controllable prisms as used in FIGS. 3b, 4a, 4b, and 5 are for
example liquid prisms, which contain two immiscible liquids whose optical
refraction behaviour can be modified by supplying a voltage.

[0077]A fourth embodiment of the invention is shown in FIGS. 6a and 6b.
The light modulator means SLM is combined with a matrix of rhombic prisms
and an active or passive polarisation switch PU. A single rhombic prism
splits each wave front into two parts which exhibit perpendicular
polarisations p-pol; s-pol, as shown in detail in FIG. 6a. This single
rhombic prism is one of multiple, regularly arranged prisms of the
matrix, which is made clear by the arrow which points at the matrix. The
polarisation switch PU, which is disposed between the light modulator
means SLM and the prism matrix, can be set to a certain angle. At an
angle of 45°, for example, it splits the modulated wave front into
two parts of same size, whereby all reconstructed object points are
displaced one-dimensionally, lateral to the original direction of
propagation.

[0078]In another embodiment (not shown), the rhombic prisms can for
example be switched periodically by 90° in order to displace the
wave fronts sequentially in one direction and to superimpose the object
points.

[0079]The combination of prism matrix and polarisation switch PU is
provided twice in order to achieve a two-dimensional displacement, as
shown in FIG. 6b. It must be noted that the second prism matrix, which is
disposed behind the polarisation switch PU, seen in the direction of
light propagation, is turned by an angle of 90°. The polarisation
switch PU turns the electric field by 45°. This arrangement allows
the modulated wave front to be divided sequentially into four identical
parts with like brightness values, which are then superimposed on the
retina.

[0080]A pattern which is achieved with this method of superimposing object
points is shown in detail in FIG. 6b, which is indicated by the arrow in
the drawing.

[0081]A λ/2 plate can be used as passive polarisation switch PU, and
a Faraday cell can be used as active polarisation switch.

[0082]In a fifth embodiment of the invention, the displacement of object
points is generally achieved by taking advantage of the birefringence
effect. In a birefringent material, two optical axes are oriented
depending on the direction, so that if the material is disposed in the
beam path in a certain position, a pencil of rays or a wave front is
split into two parts by way of refraction during its passage through the
material.

[0083]Referring to FIG. 7, the modulated wave front which comes from the
light modulator means SLM thus falls on a first optical component made of
a birefringent material. The orientation of the material is indicated by
a double arrow. Two differently polarised wave fronts, denoted with s-pol
and p-pol, exit the birefringent material in parallel. In order to
displace the two wave fronts in a different direction, e.g. perpendicular
to the former, a λ/2 plate or another polarisation switch PU is
disposed in front of a second optical component made of a birefringent
material. The λ/2 plate turns the polarisation of the two wave
fronts by 45°, so that both of them enter the second material
under that angle. After its passage through the second material, the wave
front of an object point is displaced in four directions, and the wave
fronts of this object point are accordingly superimposed four times on
the retina. The four different directions are indicated in a
non-perspective way by arrows in FIG. 7. Adjacent wave fronts on the
retina exhibit perpendicular polarisations and do not interfere with each
other, but are superimposed incoherently.

[0084]In a sixth embodiment of the invention, Bragg gratings are provided
in the reconstruction beam path of an holographic display device for
displacing the modulated wave fronts. They exhibit a similar behaviour as
a birefringent material. By choosing a certain angle of incidence and
wavelength of the laser light, the angle and wavelength of those grating
structures can be chosen variably when producing the Bragg gratings by
way of exposing a holographic recording medium. Thanks to these
characteristics, Bragg gratings are very well suited to displacing wave
fronts in a defined manner and to multiply reconstructed object points.

[0085]FIG. 8 shows the general design of a Bragg grating BG with a grating
geometry of 60°/0° and 0°/60°, for example,
where a spacer AH is provided between the grating structures. A
deflection of 60° of incident wave fronts of the laser light is
realised within the Bragg grating BG with such an arrangement. The Bragg
grating BG can here have a diffraction efficiency of 100% for one
polarisation direction and of 0% for the perpendicular component.

[0086]The thickness of the spacer AH and the geometry of the grating
(diffraction angle) determine the respective lateral displacement of an
incident wave front in relation to its original direction of propagation.

[0087]With a polarisation vector of for example 45° in relation to
the geometry of the grating, there will be two resultant wave fronts with
like brightness values. The second grating has the same geometry as the
first one, and thus diffracts only one wave front, while the other wave
front passes the grating without being affected. Both wave fronts exit
the second Bragg grating BG in parallel, so to realise a one-dimensional
displacement.

[0088]Other combinations of optimal polarisation beam splitter geometries
of Bragg gratings BG are also thinkable, for example with diffraction
angles of 45°/0° or 30°/0°, or with odd
angles.

[0089]The spacer AH can for example be a foil, a plastic plate or a glass
plate having a thickness of up to 200 μm.

[0090]FIGS. 9a and 9b show views of a solution for a two-dimensional
displacement of modulated wave fronts with Bragg gratings. Generally, the
two-dimensional displacement can either be realised by a sequential
combination of Bragg gratings or by writing a number of Bragg gratings to
a holographic recording medium. The latter is also referred to as a
volume hologram.

[0091]FIG. 9a is a side view showing a volume hologram comprising two
Bragg gratings BG. The modulated laser light, which comes from the light
modulator means SLM, and which comprises two polarisation components, s
and p, falls on the Bragg gratings BG.

[0092]The Bragg gratings BG are chosen such that the modulated wave fronts
of the polarisation component p are split into two components p during
their passage. Both components p have the same polarisation, but
propagate in a plane symmetrically in two directions.

[0093]A second volume hologram (not shown) also comprises two Bragg
gratings. During the passage of the second volume hologram, the other
polarisation component s is likewise split into two components s with
identical polarisation in another plane, which is perpendicular to the
former one.

[0094]Both p components and both s components (View A) of the volume
holograms are shown symmetrical and mirror-inverted to the optical axis
in FIG. 9a.

[0095]The two volume holograms are arranged such that the direction of
propagation of the original modulated wave fronts is maintained after
their passage through the two volume holograms.

[0096]FIG. 9b shows the front view of the volume hologram of FIG. 9a. The
representation of the second volume hologram and spacers was omitted
again in this diagram.

[0097]The Bragg gratings BG contained in the two volume holograms generate
a resultant pattern of a two-dimensionally multiplied object point on the
retina, which is indicated by the arrow in FIG. 9b. In the pattern, two
superimposed object points always have the same polarisation s and p.
Because superimposed adjacent object points are differently polarised,
they will be reconstructed incoherently. An observer eye again perceives
the resultant reconstruction of the scene with reduced speckle pattern
here.

[0098]In a seventh embodiment of the invention, a scene, which is composed
of the three primary colours RGB (red, green, blue), is reconstructed in
colour using Bragg gratings. In order to realise a multiplication of the
reconstructed object points for each colour, e.g. a combination of Bragg
gratings with a spacer is used for each colour in the reconstruction beam
path, as shown in FIG. 8.

[0099]In another physical form of the seventh embodiment, a colour
reconstruction of the scene can also be realised in that a volume
hologram contains a number of Bragg gratings for each colour. Generally,
the number of Bragg gratings here depends on the number of desired
superimpositions of the wave fronts of an object point with themselves.
The larger the number of superimpositions, the finer are individual
speckle patterns averaged by the observer eye.

[0100]Referring to FIGS. 9a and 9b and the related description, if a
colour reconstruction of the scene is generated, a volume hologram must
contain two Bragg gratings per colour and two per direction. This makes a
total number of 2×2×3=12 Bragg gratings, which are required
altogether to realise a colour reconstruction.

[0101]Because of the great angle sensitivity of the Bragg gratings, laser
light must be emitted in a very small angular range when realising the
invention. This can be achieved in that [0102]a) the means which effect
the displacements are arranged behind the light modulator means, seen in
the direction of light propagation, but in front of a field lens, which
can for example be a Fresnel lens or a diffractive optical element DOE,
or [0103]b) the means which effect the displacements are spatially
divided into an adequate number of individual Bragg gratings, where the
geometry of the Bragg gratings varies with the position of the modulated
wave fronts.

[0104]In particular the arrangements for multiplying all reconstructed
object points of the scene with themselves with the help of Bragg
gratings, as used according to the invention, do not require any active
components in the display device. The Bragg gratings are the most
efficient means of all above-mentioned means, as regards both technical
and economic viability. Their great advantages are that they do not have
to be moved mechanically, that they do not require any electronic
control, and that they can thus be manufactured as passive elements.